1. Field of the Invention
This invention relates to a digital electronic still-video camera for recording digital image data representing a subject on a recording medium such as a memory card. The invention further relates to a method of controlling such a camera.
2. Description of the Related Art
Photometry is required in order to determine exposure conditions in a digital electronic still-video camera having an automatic exposure (so-called "AE") function. Photometry can be performed in a variety of ways. One method is to dispose a photometric element on the front side of the digital electronic still-video camera. Though this method is a simple one in terms of construction, an intrinsic problem which arises is that the area in which light is measured does not coincide with the area photographed. This problem becomes particularly pronounced in a case where the field of view is changed using a zoom lens. In order to solve this problem, it is necessary to change the photometric area of the photometric element in operative association with the zoom lens of the photographic system. This necessitates an elaborate mechanism.
Another method (referred to as "TTL photometry") that is available involves incorporating the photometric element in the image pick-up optical system and making the photometric area coincide with the photographic area. With this method, the image pick-up optical system requires such devices as a beam splitter and elements for changing the optical path. This leads to a larger optical system and to a decrease in sensitivity caused by a decrease in the transmittance of light. In a case where a mirror is used, problems arise in terms of durability and reliability.
Accordingly, in the case of a digital electronic still-video camera having a solid-state electronic image sensing device (such as a CCD) in which a video signal representing the image of a subject is obtained and converted into digital image data, consideration has been given to a method in which a photometric value is found by integrating the digital image data over an appropriate photometric area. This approach is advantageous in that the photographic area and photometric area coincide completely, there is no increase in size and it is unnecessary to provide extra elements for the optical system. In addition, such variations as average photometry, partial photometry and split photometry become possible by subjecting the digital image data to data processing. Since this makes it possible to set exposure conditions corresponding to a variety of photographic environments, a broader range of applications is achieved.
Digital electronic still-video cameras further have various automatic focusing functions (so-called "AF" functions). These include a function in which an incident light image is preliminarily captured by a solid-state electronic image sensing device such as a CCD, the video image thus obtained is converted into digital image data and focusing control is performed using this digital image data.
However, in a case where exposure control and focusing control are performed using digital image data, circuitry for this purpose is necessary and it is difficult to make the digital electronic still-video camera small in size and light in weight. In a digital electronic still-video camera, it is necessary to convert the analog video signal outputted by the CCD into digital image data. However, when an analog/digital converting circuit is provided for each of recording processing, exposure-control processing and focusing-control processing, a small-size, light-weight camera cannot be achieved.